Rotary barrel diffusion coating in molten lead

ABSTRACT

A NOVEL PROCESS FOR DIFFUSION COATING OF FERROUS ARTICLES IN A LEAD BATH BY SEALING ALL OF THE CONSTITUENTS IN A BARREL WHICH IS ROTATED IN A NON-VERTICAL POSITION AT THE CHROMIZING TEMPERATURE. SUCH PROCESSING RESULTS IN EXTREMELY DIFFUSED ZONES ON IRREGULARLY-SHAPED PARTS.

United sates pat nt once 3,778,299 ROTARY BARREL DIFFUSION COATINGMOLTEN LEAD 1 John J. Rausch, Antioch, and Ray J. Van Thyne, Oak Fawn,111., assignors to Surfalloy Corporation, Chicago,

l]. 1 Drawing. Continuation-impart of application Ser. No. 768,187, Oct.16, 1968, now Patent No. 3,620,816.

This application Nov. 12, 1971, Ser. No. 198,404

- Int. Cl. C23c 9/00 US. Cl. 117-109 3 Claims ABSTRACT on THE DISCLOSUREA novel process for diffusion coating of ferrous articles in a lead bathby sealing all of the constituents in a barrel which is rotated in anon-vertical position at the chromizing temperature. Such processingresults in extremely diffused zones on irregularly-shaped parts.

CROSS REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of our copending application Ser. No. 768,187entitled, Method of Diffusion Coating Metal Substrates Using Molten Leadas Transport Medium, filed Oct. 16, 1968 now US. Pat. 3,620,816.

BACKGROUND OF THE INVENTION In our copending US. patent application(Ser. No. 768,187) now U.S. Pat. 3,620,816 there is described adiffusion coating process in which a metal article is surface alloyed bycontacting the article with molten lead containing one or more diffusingelements. This process particularly provides chromium impregnated(chromized) ferrous articles which have good resistance to corrosion. Ingeneral, the process involves immersing the article to be coated in amolten lead bath for a predetermined time, removing the diffusion coatedarticle from the bath, and thereafter cleaning the article to removeadherent lead and bath residue. In order for the process to be carriedout more effectively we have found that some degree of agitation of theparts and the bath must occur. It is desirable that the elements thatare being transferred to the ferrous article to be maintained at someeffective degree of solution concentration in the lead. It is alsodesirable that the ferrous articles not remain in contact with eachother for prolonged time periods since the lead solution containing thetransfer elements can not effectively maintain saturation and causetransfer under such conditions. Furthermore, said ferrous articles .willtend to bond together if allowed to reside in contact for prolonged timeperiods in the bath. It should also be pointed out that ferrous articleswill float in a lead". bath and in order for diffusion coating to occursuch articles must be maintained below the surface of the bath at leastduring most of the diffusion coating cycle. We have also found that itis desirable to exclude air or other gases that will react with thelead, chromium, or other ingredients of the bath. Either vacuum or aninert gas such as argon, for example, are suitable covers for the bath.However, if the contained space above the bath is large the impuritiespresent in the atmosphere can cause significant contamination of thebath. Furthermore, if that space is large a significant amount of leadis lost by evaporation. Thus it is most desirable to have the bathsealed with only a small residual space such that very littlecontamination or evaporation can occur. If this space is small enougheven air can be left in the space since it will not contaminate the bathsignificantly.

We have discovered an effective means of diffusion coating with our leadbath process that eliminates all of the problems heretofore described.In this method, all of 3,778,299 Patented Dec. 11, 1 973 DESCRIPTION OFTHE INVENTION When the process is used for chromizing, the ferrousparts, chromium source and other diffusing elements are charged into thebarrel which is heated to a temperature in the range 600 to 800 F. Thebarrel is continuously purged with argon prior to and during the fillingcycle to avoid oxidation of the materials. Molten lead is poured intothe barrel until about of the volume is filled With lead and the othermaterials. The barrel is then sealed by welding or it is preheated,while the inside is either evacuated or purged with an inert cover gas,to a temperature near the chromizing temperature (-1950 F.) and thensealed with an external flange. The flange is normally locatedsufficiently far from the vaned barrel (the reaction chamber) such thata gasket, Teflon, for example, can be used for sealing. The externalflange is connected to an insulated plug having a diameter slightly lessthe ID. of the barrel. If the plug is of sufficient length such that theflange is below the melting point of lead it is possible to use solidlead, which will form when the barrel is placed in a horizontalposition, as a sealant.

The barrel can be made of iron, or an alloy of iron that contains thediffusing elements being transferred, provided those elements are notextensively soluble in lead. For chromizing, we frequently use Fe-Crstainless steels such as A.I.S.I. type 446 (Fe-25 Cr). Under theseconditions the chromium in the alloy will not be dissolved significantlysince the chromium source in the bath will have a higher chromiumcontent. Nickel-base alloys cannot be used, without an intermediateliner, since nickel is readily dissolved by lead.

For 4 in. ID. barrels we normally use 4 vanes, l in. long which areattached radially and spaced 90 apart. Barrels of this type have beenrotated at speeds ranging from k to 6 rpm. and have resulted in veryuniform chromizing of all of the parts in the barrel. For largerdiameter barrels more numerous and longer vanes could be used toaccomplish effective bath stirring and relative motion of the parts.

The reliability of our rotating barrel process has been demonstrated bymany examples. Over 35 separate runs have been made using ferrous partswith internal holes and threads. Typically the chromizing treatmentswere 1950 F. for 4 hours, which produces a chromized layer about 0.003inch deep on very low carbon-containing parts. Of course othertreatments may be used depending upon the desired chromizing zone depth.Over parts that were selected at random have been sectioned andmetallographic examination at 100x magnification showed that in everyexample the chromized layer was uniform.

We have successfully used both ferrochromium and un-alloyed chromium. Inaddition to chromium per se, the process can be used to diffuse otherelements along with chromium or to diffuse other elements, soluble inlead, in the absence of chromium. Examples of the latter includealuminum, titanium, silicon, cobalt, nickel, and others. Our barrels arenormally operated in a horizontal position but they could be operated inother non-vertical positions, up to 60 degrees from the horizontal, withsuitable vaning designs.

We claim as our invention:

1. A method of diffusion coating ferrous articles using a lead bathcontaining a source of the diffusing elements comprising the steps of(a) providing a molten lead bath containing the elementor elements to bediffused;

(b) placing said bath and said ferrous articles in a sealable, vanedcylindrical barrel; and filling said barrel therewith to minimize freespace over said bath;

(c) sealing said barrel;

(d) rotating said barrel in a position ranging from horizontal to 60degrees from the horizontal While maintaining the bath therein at amolten temperature and while rotating maintaining said ferrous articlessubstantially submerged within said bath; and

(e) maintaining said rotation for an adequate time to diffusion treatsaid ferrous articles.

2. A method as defined in claim 1 in which the difiusing element ischromium.

3. A method as defined in claim 1 wherein the barrel is held in androtated in a horizontal position.

References Cited UNITED STATES PATENTS 3,377,195 4/1968 Sneesby 117114 R2,793,965 5/1957 Myers et al. 117109 2,864,731 12/1958 Gurinsky et al.1486.l1 2,929,741 3/1960 Steinberg 117-118 X 135,339 1/1873 Hunt 117-109X 1,713,233 5/ 1929 Kelley 1171 14 BX 2,910,379 10/1959 Gurinsky 117118X 2,926,111 2/ 1960 Schweitzer et al. 1486.11

15 RALPH s. KENDALL, Primary Examiner US. 01. X.R. 117-414 R, 119

